1. Field of the Invention
This invention is directed to the dimerization of olefins comprising from 2 to 4 carbon atoms, e.g. propylene, wherein said dimerization is performed in the presence of a catalyst of gamma and/or eta alumina having been impregnated with nickel sulfate and calcined at a temperature as low as 750.degree.F to as high as 1150.degree.F in an inert oxygen-free non-reducing atmosphere, e.g. helium, nitrogen and combinations thereof. The amount of nickel impregnated on said alumina is critical, with decreased activity observed when the nickel content of the catalyst is less than 2 weight percent and with decreased selectivity observed when the nickel content of the catalyst is more than 5 weight percent. The most advantageous results are obtained when the nickel content of the catalyst is between about 2 weight percent and about 3 weight percent. Said catalyst, when used in the process of the present invention, is characterized by exceptionally long catalyst life, capability of affording high yields of desired dimer products with high selectivity, e.g. propylene dimer (C.sub.6 .sup.-) when propylene is the feed olefin, and being easily and effectively regenerated, when necessary, without substantial loss in activity. The specific catalyst for use herein is stable through multiple regenerations.
2. Discussion of the Prior Art
Dimerization and polymerization of compounds such as ethylene and propylene using catalysts other than that of this invention is known in the art. For example, an article appearing in the April, 1955, issue of Industrial and Engineering Chemistry, Volume 47, pages 752-757, disclosed that a catalyst of nickel oxides impregnated on silica-alumina may be useful for the polymerization of light olefins, such as those contained in a refinery cracked gas, e.g. ethylene. The article further disclosed that active catalysts were obtained by impregnation of the silica-alumina with solutions of nickel chloride or nickel sulfate. The article emphasized, however, that the most active polymerization catalyst was prepared by using an impregnation solution containing about 40 weight percent nickel nitrate hexahydrate to impregnate the silica-alumina.
Unfortunately, in the process of dimerization of light olefins as herein set forth, alumina impregnated with nickel nitrate has no activity (demonstrated by example hereinafter).
Also a Russian article appearing in Zhurnal Organicheskoi Khimii, Vol. 8, No. 3, pages 650-651 (March, 1972), disclosed the use of nickel chloride deposited on an aluminosilicate or aluminum oxide, which has been activated by heating under vacuum or inert gas, for dimerization of ethylene and propylene. In such a process, however, it is known (and shown by specific example hereinafter) that the catalyst ages rather rapidly with regeneration causing substantial decline in catalyst activity. Further, the above nickel chloride catalyst requires a minimum of about 950.degree.F for activation, whereas the catalyst for use herein may be activated at lower temperatures of, for example, as low as 750.degree.F.
U.S. Pat. No. 2,794,842 teaches catalytic polymerization of olefins in the presence of nickel sulfate-promoted catalysts. That patent teaches the necessity in its process of activating the catalyst for use therein by heating in an oxygen-containing atmosphere. It further teaches the catalyst for use therein as having from 0.1 to 10 weight percent nickel. Applicant has discovered that the use of the present catalyst, i.e. gamma and/or eta alumina being intimately combined with a very limited range of nickel, e.g. 2 to 5 weight percent, and sulfur, e.g. 1 to 3 weight percent, as a result of impregnation with nickel sulfate in appropriate amount and being calcined at a temperature of from 750.degree.F to 1150.degree.F in an inert oxygen-free, non-reducing atmosphere, provides high conversion of olefin feed with high selectivity to dimer. This, of course, is at the expense of polymer formation in the process. By the present process, alkenes are produced having exactly twice the number of carbon atoms and hydrogen atoms as the alkene feed in high selectivity and conversion. If an oxygen-containing or a reducing atmosphere is used in calcining the catalyst or if the catalyst contains less than 2 or more than 5 weight percent nickel, lower conversion and/or selectivity results.
U.S. Pat. No. 3,557,242 teaches a process for dimerization of olefins using a silica-alumina catalyst incorporating nickel oxide or a nickel salt, wherein the proportion of alumina in the catalyst is within the range of from only 0.1 to 10 percent by weight. That patent teaches that increasing the alumina content of the catalyst over the 10 percent maximum destroys dimer selectivity. Applicant has discovered that dimer selectivity is enhanced when the catalyst employed is a 100 percent alumina structure intimately combined with nickel and sulfur as above described.